1. Field of Application
The present invention relates to an inductive load drive apparatus for applying PWM (pulse width modulation) control to mutually separately drive a pair of inductive loads that are connected to a DC power source.
2. Description of Prior Art
In the prior art, various methods have been proposed for overcoming the problems that arise when a plurality of loads are driven in parallel, with the driving being based on PWM control signals. For example with Japanese patent 2002-43910, it is ensured that each falling edge of a PWM control signal that controls driving of one of the loads coincides with a rising edge of the PWM control signal that controls driving of the other load, for thereby ensuring that an increase in load current of one of the loads will be cancelled by a decrease in load current of the other load, and so reduce variations in the overall load current. That is to say, if both of the loads were to be supplied with load current concurrently at any time, then a large increase in the overall load current would occur. However with that invention, by establishing a shift between the conduction timings of the two loads, it is ensured that the degree of variation of the overall load current is minimized.
Such a technique has been successfully applied in the case of resistive loads, such as the headlamps of a vehicle. However it would be desirable to be able to apply a similar method to driving inductive loads, such as electric motors. When a switching element such as a FET (field effect transistor) is used to control current flow through an inductive load from a DC power source such as a battery, with a rectangular-waveform PWM control signal being applied to the switching element, then each time the switching element is set in the off state, a regenerative (i.e., reverse-polarity) current flow occurs from the inductive load back into the power source. As a result, the waveform of power source current flow is approximately sinusoidal.
FIG. 7 is a comparative example for illustrating how it might be attempted to adapt the technique of the above-mentioned Japanese patent to the case of driving a pair of inductive loads, which will be assumed to be respective motors. In FIG. 7, a series combination of a motor 2A and MOS FET 2A and a series combination of a motor 2B and MOS FET 2B are connected in parallel between the potential of a DC power source consisting of a battery 1 and ground potential. More specifically, the drains of the MOS FETs 2A, 2B are connected through respective diodes 4A, 4B to one side of a π-configuration filter (referred to in the following simply as a π filter) 5, with the other side of the π filter 5 connected to the potential of the battery 1, and with the diodes being connected in a direction such as to be reverse-biased when the corresponding MOS FET is set in the on state. (It should be noted that neither the diodes 31A, 31B nor the π filter 5 are described in the aforementioned Japanese patent 2002-43910, which describes the driving of resistive loads only).
When a MOS FET 3A or 3B is switched from the on to the off state, a resultant regenerative current flows from the corresponding one of the motors 2A, 2B through the corresponding one of the diodes 4A, 4B into the battery 1. The π filter 5, which is formed of a coil 8 and capacitors 6, 7 as shown, serves to absorb these flows of regenerative current and thereby smooth out fluctuations in the potential of the battery 1 which would otherwise result from such flows of regenerative current.
A pair of PWM control signals A and B having identical duty ratio, produced from a control IC 9, are applied through respective drive circuits 10A, 10B as respective PWM to the gates of FETs 3A, 3B respectively. When a FET 3A or 3B is set in the on state, current flows from the battery 1 through the corresponding one of the motors 2A, 2B and that FET to ground potential. When that FET 3A or 3B is then set in the off state, a delayed current flows through the corresponding one of the diodes 4A, 4B to the π filter 5, to return to the battery 1 as a regenerative current. This regenerative current flow is smoothed by the capacitor 6 of the filter 5. The capacitor 7, on the opposite side of the π filter 5 from the diodes 4A, 4B, serves to smooth fluctuations in the potential of the battery 1.
However with such a configuration, the level of the regenerative current is substantial, so that it is necessary to use a large value of capacitance for the capacitor 6. In order to minimize the necessary capacitance of the capacitor 6, it is desirable to reduce the peak level of the regenerative current as far as possible.
FIGS. 8A, 8B illustrate the effect of changing the phase difference between the PWM control signals A and B upon the waveform of a ripple component which appears in the supply current of the battery 1 with the circuit of FIG. 7, for the case in which the PWM control signals are of approximately rectangular waveform. The ripple component is measured at the positive terminal of the battery 1.
FIG. 8A shows the power supply voltage waveforms for MOS FETs 3A, 3B as measured at points A, B in FIG. 7, when the PWM control signals are of identical phase. In that case the motors 2A, 2B are supplied with power concurrently, and hence a large amplitude of ripple appears in the power source current from the battery 1. However if a suitable phase difference is established between the control signals A and B, such as to ensure that the motors 2A, 2B are not supplied with current simultaneously from the battery 1, as shown in FIG. 8B, then the amount of ripple can be substantially reduced.
A problem arises with such a method whereby the motors 2A, 2B are driven alternately as shown in FIG. 8B, in that the frequency of the ripple component in the supply current of the battery 1 is doubled, by comparison with the case in which the motors are driven concurrently. As a result, electrical noise is generated at the frequency of that ripple component. The overall noise level of the vehicle electrical system is thereby increased.
It might be envisaged that this problem could be overcome by forming the control signals with a trapezoidal waveform and arranging that the start of each rising edge of one of the control signals A and B is made to coincide with the start of a falling edge of the other one of these signals, as illustrated in the timing diagram of FIG. 9A. It would appear that such a method could substantially reduce the level of the ripple component. However in practice, due to the intervals in which both of the FETs 3A, 3B are conducting simultaneously, that is to say when a turn-on interval (i.e., transition interval from the non-conducting to the conducting state) of one FET overlaps a turn-off interval (i.e., transition interval from the conducting to the non-conducting state) of the other FET as illustrated in FIG. 9A, distortion of the waveform of the ripple component of the supply current of the battery 1 is produced. This waveform distortion increases the level of electrical noise.
Another problem which arises in the prior art with respect to driving a plurality of motors constituting respective inductive loads is as follows. In certain applications, such as driving the cooling fans which direct air flows into the engine radiator and into the condenser of the air conditioner system of a motor vehicle, it is desirable that identical rates of air flow are produced by each of the cooling fans, in order to maximize the efficiency of the cooling operation. Thus, for example in the case of using two motors in parallel in such an application, it would be preferable to be able to utilize two motors which have identical values of power rating, for example which are both rated at 100 W, or are both rated at 200 W, to ensure that identical levels of power are produced by the motors when they are driven by respective PWM voltages of identical duty ratio. However it might be necessary that the maximum amount of output power that will be produced (in total) by the motors is for example to be 260 W. In such a case, since it is very possible motors having a power rating of 130 W may not be available, it might in practice be necessary to use a pair of motors which have respectively different values of power ratings, for example a combination of a 100 W motor and a 160 W motor. However with prior art methods of PWM drive control of such a combination of motors by using a common value of PWM duty ratio for both motors (and so, identical levels of average drive voltage) this would result in unbalanced amounts of output power being produced by the motors. Such cases of having to utilize an unbalanced combination of motors are very frequent in the prior art, and lead to inefficiency in such applications as driving the cooling fans of a vehicle as described above.